A friction material for a brake is produced by binding an organic powder material (e.g. cashew nut shell dust and rubber powder), an inorganic powder material (e.g. barium sulfate, iron oxide and graphite) or metal powder (e.g. copper powder and iron powder) together with a fiber material (e.g. asbestos, metal fiber, ceramic fiber, glass fiber and aramid fiber) with a thermoset resin such as phenol resin followed by molding and curing.
When the brake is abruptly heated to a high temperature by, for example, continuous use, so-called fading is caused, this resulting in decrease of braking effect. "Automobile Technology" (Jidosha Gijutsu), Vol. 27, No. 2, (1973) 148-156 describes conventional technologies and fading of a brake. According to this literature, one of the main causes of fading of the brake is attributed to a gas, which is generated by decomposition of the organic material contained in the friction material by frictional heat and present on a friction surface of the brake, which reduces the apparent coefficient of friction. Several measures have been proposed to increase fade resistance of the friction material. Such measures include increases of porosity of a pad to form minute conduits for ventilating the gas, and scorching wherein the pad is thermally treated to evaporate volatile materials.
The porosity of the pad can be increased by decreasing an amount of the binder resin or by reducing molding pressure. However, both of these procedures deteriorate wear resistance of the friction material although it can increase the porosity. The scorching of the material not only raises production costs due to increase of the number of production steps but also requires a troublesome treatment of the generated gas. Further, the scorching is associated with decrease of the wear resistance of the friction material due to thermal deterioration.
According to the conventional art, wear of the friction material is increased when the porosity is increased so as to improve the fade characteristics of the friction material. A reason for this is that a binding structure of the whole material is roughened by the reduced molding pressure or the decreased amount of the binder resin. If a microstructure of the friction material could be made tough and dense and micropores through which gas passes could be formed in the structure, the fade resistance of the friction material would be improved with keeping the decrease of the wear resistance at a minimum level.
Such friction material could be produced by molding a tough and dense friction material with using a sufficient amount of the binder resin under high molding pressure and thereafter forming bores through the friction material over the whole surface thereof by means of a minute drill. However, such manner is not suitable for practical and industrial production of the friction material.